Usually a rigid polyurethane foam has closed cells, and a gas such as a halogenated lower aliphatic hydrocarbon, e.g., fluorotrichloromethane, or carbon dioxide used as a blowing agent is enclosed in the cells. Since such a gas has a low thermal conductivity, the rigid polyurethane foam is suitably used as a heat insulating material in, for example, refrigerators, buildings, low temperature warehouses, storage tanks, refrigerator ships, or pipings. However, such a conventional rigid polyurethane foam as above mentioned is limited in use since the foam readily changes in dimensions, or is deformed remarkably under the circumstances where tempratures change sharply.
In contrast, an open cell rigid polyurethane foam is not so good as the closed cell rigid polyurethane foam in heat insulating properties, however, it has an advantage that it can be safely used without any substantial dimensional change under the circumstances where temperatures change sharply.
There have hitherto been proposed a number of methods for the production of open cell rigid polyurethane foam, among which is, for example, a method wherein a variety of polyols having a variety of functionality are used in combination, as described in Japanese Patent Publication No. 54-5840 or No. 4-487.
However, the method employs a halogenated hydrocarbon together with a small amount of water as a blowing agent, so that the resultant foam has in part cell walls of a reduced mechanical strength and the walls are partly broken by the gas heated when the foam is formed, thereby to provide an open cell rigid polyurethane foam. Accordingly, the method allows the halogenated hydrocarbon or blowing agent to be released into the air when the foam is produced, with a result that such a gas might cause environmental contamination. In addition, such release of gas is disadvantageous from the economical standpoint. As a further problem, the halogenated hydrocarbon used as a main blowing agent in the prior art is poorly miscible with water, and thus the mixture of the halogenated hydrocarbon and water tends to separate, so that the use of such a mixture as a blowing agent fails to provide an open cell foam having a fixed quality in a stable manner.
As set forth above, there has been developed no method which produces an excellent open cell rigid polyurethane foam in a stable manner by use of a blowing agent substantially comprising water, since water tends to react with an isocyanate compound to form urea bonds thereby to form crosslinking structures in the resultant polyurethane foam, as well as to produce carbon dioxide.
Moreover, in general, when water is used as the main blowing agent, an open cell rigid polyurethane foam may be produced by a free foaming process, however, a closed cell rigid foam may be readily produced on account of an imbalance of foaming reaction and crosslinking reaction either by a foam casting method wherein a mold is used or a spray foaming process wherein the material used has a high reactivity. It is also difficult to prepare a uniform premix since the components used therein are poor in miscibility with each other when water is used as a main blowing agent.